This application is based on application No. 241359/1999 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a toner used for developing electrostatic latent images in an image-forming method such as an electrophotographic method, an electrostatic recording method and an electrostatic printing method, and more specifically concerns a toner preferably used for developing electrostatic latent images in a full-color image-forming apparatus such as a full-color electrostatic copying machine and a full-color laser beam printer.
2. Description of the Related Art
In the case of a full-color image-forming system using the electrophotographic method, since a full-color image is obtained by superposing toner images of magenta toner, cyan toner and yellow toner, it is one of the greatest requirements for the toner to have sharply-melting properties so as to allow the toner layers of the respective colors to melt instantaneously by heat, to be able to be mixed easily and to allow for clear color developments, at the time of fixing (U.S. Pat. Nos. 4,142,982, 4,590,139); however, toners of this type have a high viscosity, low elasticity and a small particle-to-particle aggregating force at the time when the toner is thermally fused, resulting in a problem of off-set with the heating roller (particularly, at the time of high temperatures). Such a problem becomes conspicuous especially when there is degradation in the roller due to repetitive use and when an image having a large amount of toner or a large toner adhering area (for example, a photograph image) on a toner recording material (such as paper). For this reason, in order to additionally improve the anti-off-set property at high-temperatures, mold-releasing oil, such as silicone oil, has to be applied to the fixing roller; however, this causes another problem of too much luster caused by the oil.
In recent years, as full-color printers and copying machines with high image quality have come to be widely used, the number of output sheets of color images have increased drastically, and there have been increasing demands for high speed operations and toners that are less susceptible to changes in image luster at the time of continuous copying processes. Great variations in luster tend to make the same color look different, and variations in luster give great influences on hues and color reproducibility sensed by the human eye.
However, in the case when a conventional toner that puts an emphasis on the sharp melting properties is used, there is a problem in which the image luster changes greatly at the time of continuous copying processes. This problem becomes more conspicuous when the copying speed becomes faster. It is considered that the variations in luster occur due to a reduction in the fixing roller temperature that gradually takes place as paper takes heat from the surface of the fixing roller. Moreover, when, after continuous copying processes have been carried out by using A-4 paper longitudinally, the change to a lateral A-4 copying process leads to variations in luster occurring in an image on a single sheet, resulting in another problem. More specifically, when A-4 paper is used longitudinally, portions on both ends of the fixing roller do not contribute to the fixing process, while in the case when A-4 paper is used laterally, not only the center portion of the fixing roller, but also both ends of the fixing roller, contribute to the fixing process, with the result that a temperature difference occurs between the center portion and both ends of the fixing roller at the time of copying, thereby causing portions with high luster and other portions with low luster. Such problems of variations in luster at the time of continuous copying processes and variations in luster on one copied image become more conspicuous in the case of the processes at cold places and immediately after power application.
The following patent applications have disclosed techniques in which the viscoelasticity of a toner or a resin used therewith has been taken into consideration: for example, Japanese Patent Laid-Open No. Hei 5-142963, Japanese Patent Laid-Open No. 8-101530, U.S. Pat. No. 5,707,771, Japanese Patent Laid-Open No. 8-334930, U.S. Pat. Nos. 5,753,399, 6,002,903, 5,766,816 and 5,840,457.
However, some of these techniques fail to properly find a state at the time of paper separation in which the luster is actually determined due to the fact that the viscoelasticity-measuring temperature is too high, or some of them fail to provide an image with proper luster since the specified value is too high. In other words, any of the toners in the above-mentioned techniques fail to provide an image with proper luster when the fixing roller temperature varies or any temperature difference occurs in the fixing roller due to variations in ambient conditions or continuous copying processes for many copies.
The present invention is to provide an electrostatic-image developing toner which can provide an image with proper luster even when the fixing roller temperature varies or any temperature difference occurs in the fixing roller due to variations in ambient conditions or continuous copying processes for many copies, and which also has superior anti-offset properties.
Another objective of the present invention is to provide an electrostatic-image developing toner which is also superior in the low-temperature fixing properties and the anti-blocking properties.
The above objects can be achieved by a toner comprising:
a first binder resin and a colorant;
wherein the toner has a temperature range of 90 to 110xc2x0 C. at 2xc3x97103 (Pa) of a storage elastic modulus (Gxe2x80x2) under 0.1 (Hz) of a frequency.
The present invention provides a toner comprising:
a first binder resin and a colorant;
wherein the toner has a temperature range of 90 to 110xc2x0 C. at 2xc3x97103 (Pa) of a storage elastic modulus (Gxe2x80x2) under 0.1 (Hz) of a frequency.
The inventors of the present invention have focused their attention to the toner behavior upon fixing toner on a recording material (more specifically, at the time when toner (image) is separated from the fixing roller upon fixing the toner on the recording material by using the fixing roller), and have found that the luster of an image depends on the smoothness of the surface of the image, that the smoothness of the surface of the image depends on the adhering force exerted between the surface of the image and the fixing roller, that the adhering force depends on the behavior of the toner (elastic characteristic) as an elastic member, that when the storage elastic modulus (hereinafter, referred to simply as xe2x80x9cGxe2x80x2 xe2x80x9d), which serves as an index for the corresponding elastic characteristic, is set at 2xc3x97103 (Pa), the surface state (smoothness) of an image has an optimum image luster (degree of luster from 15 to 40), and that the temperature at this time is located in a specific temperature range, an image having a superior luster is obtained through a low-temperature fixing process even when the temperature of the fixing roller is varied.
More specifically, in the case when the smoothness of the image surface is too low, the incident light is diffused due to fine irregularities of the surface, with the result that the luster becomes insufficient, and in the case when the smoothness of the image surface is too high, the incident light has an excessively high and strong reflectivity (which gives too much gloss to the entire image). Moreover, when the adhesive strength between the image and the contact face of the fixing roller is too small, the smoothness becomes higher since the image is easily separated, and when the adhesive strength is great, the separation is difficult, with the result that scratch lines appear on the separation surface and the smoothness becomes low due to the resulting irregularities on the image surface. Moreover, the adhesive strength between the image surface and the fixing roller is determined by the wettability and affinity between the two as well as the balance of the recovering strength (elastic characteristic) caused by the elastic repulsive force of the toner layer. The adhesive strength is highly dependent on the recovering strength caused by the elastic repulsive force of the toner layer.
The toner of the present invention is designed so that the temperature range that allows the storage elastic modulus (Gxe2x80x2) to have 2xc3x97103 (Pa) when the frequency is 0.1 (Hz) is set from 90 to 110xc2x0 C., preferably from 95 to 110xc2x0 C. When the temperature is less than 90xc2x0 C., the luster of the resulting image becomes too high, giving too much gloss to the image, as well as causing toner adhesion to the fixing roller, resulting in a phenomenon in which the adhering toner again adheres to the recording material such as paper (hereinafter, referred to as xe2x80x9coff-setxe2x80x9d) at comparatively high fixing temperatures. In contrast, when the above-mentioned temperature exceeds 110xc2x0 C., the luster of the resulting image becomes insufficient, the off-set occurs at comparatively low fixing temperatures, and the fixing strength of the resulting image to the recording material becomes too low, making it easy to separate the image. In this manner, in the present invention, the temperature range that allows the storage elastic modulus (Gxe2x80x2) to have 2xc3x97103 (Pa) when the frequency is 0.1 (Hz) is controlled to a specific temperature range that is comparatively low so that it is possible to provide an image with proper luster even when the fixing roller temperature varies or any temperature difference occurs in the fixing roller due to variations in ambient conditions or continuous copying processes for many copies, to improve anti-off-set properties, and consequently to reduce the amount of the application of the oil to the roller at the time of the fixing process, as well as enabling a fixing process at low temperature. Moreover, the toner of the present invention is superior in the heat resistant properties so that it is possible to avoid aggregation even at the shelf time at high temperatures (anti-blocking properties).
In the present specification, the storage elastic modulus (Gxe2x80x2) indicates the elasticity term in the viscoelastic function of a substance, and is an index that shows the degree of elasticity of the substance. The greater the value of Gxe2x80x2, the stronger the tendency of the measured substance to behave as an elastic material; in contrast, the smaller the value of Gxe2x80x2, the stronger the tendency of the measured substance to behave as a viscous material. Moreover, with respect to the temperature at which the storage elastic modulus (Gxe2x80x2) at a frequency of 0.1 (Hz) becomes 2xc3x97103 (Pa), values measured by using a viscoelasticity measuring device (Rheometer; Stresstech type, DynAlyser(copyright) DAR-100; made by ReoLogica Corp.) were adopted.
Measuring tool; parallel plate having diameter 20 mm
Measuring frequency; 0.1 Hz
Measured distortion; Max 5%
Measuring temperature; raised from 60xc2x0C. to 200xc2x0 C. at a ratio of 2xc2x0 C. per min.
More specifically, first, the toner is put on a lower plate maintained at 130xc2x0 C., and after this state was maintained for five minutes so as to sufficiently fuse the toner, an upper plate is placed so as to set the thickness of the toner at 1.0 mm, and excessive toner was removed, and this is cooled to room temperature. After the cooling process, the variation of Gxe2x80x2 in response to the temperature is followed under the above-mentioned conditions so that the temperature at which Gxe2x80x2 has a value of 2xc3x97103 (Pa) is obtained. Gxe2x80x2 does not depend on the material, surface precision, etc. of the plate.
Moreover, the toner of the present invention has small temperature dependence in the degree of luster within a range of the degree of image luster that is considered to be appropriate. In other words, in the toner of the present invention, the width of fixing temperatures that exhibit a degree of luster in the range of 15 to 40 is not less than 15xc2x0 C., preferably not less than 20xc2x0 C. In this manner, since the width of the fixing temperatures exhibiting the range of the appropriate degree of luster is wide, it is possible to provide an image with proper luster even when the fixing roller temperature varies or any temperature difference occurs in the fixing roller due to variations in ambient conditions or continuous copying processes for many copies. The problems with variations in luster of an image are considered to occur because of the fact that the temperature dependence in the degree of luster within a range of the degree of image luster that is considered to be appropriate is so high that the degree of image luster deviates from the appropriate range due to variations in the temperature of the fixing roller. In general, as the fixing temperature rises, the resulting degree of image luster increases.
In the present specification, with respect to the degree of image luster, values measured by a luster meter (GM-060; made by Minolta K.K.) were adopted; however, the above-mentioned device is not necessarily used. The range of the luster degree that is considered to be appropriate differs depending on a measuring device; therefore, when another measuring device is used, the width of the fixing temperatures exhibiting an appropriate range in the degree of luster in the corresponding measuring device is preferably set so as to be maintained in the above-mentioned range.
The toner of the present invention contains, at least, a first binder resin and a colorant, and may contain a mold-releasing agent, a charge-control agent and other agents, if necessary.
With respect to the binder resin constituting the toner of the present invention, any conventionally known resin in the field of electrophotography may be used as long as the toner as described above is obtained; and, for example, polyester resin, polystyrene resin, vinyl resin, epoxy resin, polyetherpolyol resin, etc. may be used.
With respect to the binder resin, it is preferable to use two kinds of resins (first binder resin: low molecular weight material, second binder resin: high molecular weight having different molecular weights. More specifically, a first binder resin having a number-average molecular weight (Mn) in the range of 2,000 to 8,000, preferably 2,500 to 6,000, more preferably 3,000 to 6,000, and a weight-average molecular weight (Mw) in the range of 7,000 to 30,000, preferably 8,000 to 25,000, more preferably 9,000 to 20,000, and a second binder resin having an Mn in the range of 3,000 to 12,000, preferably 4,000 to 10,000, more preferably, 4,000 to 7,000, and an Mw in the range of 30,000 to 250,000, preferably 40,000 to 250,000, more preferably 40,000 to 100,000, are used in combination. In this case, it is preferable for the two first and second binder resins to have a compatibility with each other. The application of the first binder resin is preferable from the viewpoint of toner-preserving properties at high temperatures (anti-blocking properties), the fixing strength of the toner to the recording material and the resin strength, and the application of the second binder resin is preferable from the view point of the reducing effect on the luster variation to the temperature variations, the anti-high-temperature off-set at the time of fixing and the fixing strength of the toner to the recording material.
With respect to the number-average molecular weight (n) and the weight-average molecular weight (Mw), measurements were made by using a gel permeation chromatography (GPC) (807-IT Type: Nippon Bunko Kogyo K.K.) in which: 1 kg/cm2 of tetrahydrofuran was flowed as a solvent while the column was maintained at 40xc2x0 C., and 30 mg of a sample to be measured was dissolved in 20 ml of tetrahydrofuran, and then, 0.5 mg of this solution was introduced together with the carrier solvent; thus these molecular weights were measured based upon polystyrene conversion.
In the present invention, the first binder resin has a softening point (Tm) in the range of 75xc2x0 C. to 130xc2x0 C., preferably 80xc2x0 C. to 125xc2x0 C., more preferably 85xc2x0 C. to 115xc2x0 C., and a glass transition temperature (Tg) in the range of 45xc2x0 C. to 85xc2x0 C., preferably 50xc2x0 C. to 80xc2x0 C., more preferably 55xc2x0 C. to 75xc2x0 C.; these temperature ranges are preferable from the viewpoint of the anti-blocking properties of the toner, the fixing strength of the toner to the recording material and the color mixing properties (color reproducibility) of respective toners. The second binder resin has a Tm in the range of 105xc2x0 C. to 155xc2x0 C., preferably 110xc2x0 C. to 150xc2x0 C., more preferably 110xc2x0 C. to 135xc2x0 C., and a Tg in the range of 55xc2x0 C. to 85xc2x0 C., preferably 60xc2x0 C. to 85xc2x0 C., more preferably 60xc2x0 C. to 75xc2x0 C.; thus, these temperature ranges are preferable from the view point of the reducing effect on the luster variation to the temperature variations, the anti-high-temperature off-set at the time of fixing and the fixing strength of the toner to the recording material.
In the present specification, the softening point (Tm) of resins was measured by the following method. A sample to be measured (1.0 g) was weighed, and a flow tester (CFT-500: made by Shimadzu K.K.) was used in which: measurements were made under conditions of the application of a die having a size of h 1.0 mmxc3x97xcfx861.0 mm, a temperature rise rate of 3.0xc2x0 C./min, a pre-heating time of 180 seconds, a load of 30 kg, and a measuring temperature range of 60 to 160xc2x0 C., and the temperature at the time of the 1/2 flow of the above-mentioned sample was defined as the softening point (Tm).
The glass transition point (Tg) of resins was measured by the following method. A differential scanning calorimeter (DSC-200: made by Seiko Instrument K.K.) was used in which: 10 mg of a sample to be measured was precisely weighed, and this was put into an aluminum pan, while xcex1-alumina was put into an aluminum pan so as to be used as reference, and was heated to 200xc2x0 C. from normal temperature at a temperature-rise rate of 30xc2x0 C./min, and this was then cooled, and subjected to measurements in the range of 20xc2x0 C. to 120xc2x0 C. at a temperature-rise rate of 10xc2x0 C/min; thus, during this temperature-rise process, the shoulder value of the main heat-absorption peak in the range of 30xc2x0 C. to 90xc2x0 C. was defined as the glass transition point Tg.
When the above-mentioned first binder resin and the second binder resin are used as the binder resins, the weight ratio (A:B) of the first binder resin and the second binder resin is set in the range of 5:95 to 95:5, preferably 20:80 to 95:5, more preferably 60:40 to 90:10; this setting is preferable from the viewpoint of the toner fixing properties at low temperatures, the reducing effect on the luster variation to the temperature variations and the anti-high-temperature off-set at the time of fixing.
In the present invention, it is preferable to use polyester resins as the above-mentioned first binder resin and the second binder resin. More specifically, monomers constituting the polyester resins of the first binder resin and the second binder resin are not particularly limited; and any of known acid monomers and alcohol monomers may be used.
With respect to the acid monomers, not particularly limited as long as they contain not less than two carboxylic groups, the following monomers are, for example, listed: fumaric acid, maleic acid, maleic anhydride, phthalic acid, terephthalic acid, isophthalic acid, malonic acid, succinic acid, glutaric acid, dodecenyl succinic anhydride, n-octyl succinic acid, n-dodecyl succinic acid, 1,2,4-benzenetricarboxylic acid, 1,2,4-cyclohexanetricarboxylic acid, 1,2,4-naphthalenetricarboxylic acid, 1,2,5-hexanetricarboxylic acid, 1,3-dicarboxyl-2-methyl-2-methylenecarboxylpropane, tetra(methylenecarboxy)methane, 1,2,7,8-octanetetracarboxylic acid, trimellitic anhydride, trimellitic acid, pyromellitic acid, pyromellitic anhydride, and low alkyl esters of these acids. Among these, preferable acid monomers include, for example, fumaric acid, terephthalic acid, trimellitic anhydride, succinic acid, etc. Not less than two kinds of acid monomers may be used in combination.
With respect to alcohol monomers, not particularly limited as long as they contain not less than two hydroxyl groups, the following monomers are, for example, listed: ethylene glycol, propylene glycol, 1,4-butanediol, 2,3-butanediol, diethylene glycol, triethylene glycol, 1,5-pentanediol, 1,6-hexanediol, neopentene glycol, 1,4-cyclohexanedimethanol, dipropylene glycol, polyethylene glycol, polypropylene glycol, bisphenol A and derivatives thereof, hydrogenised bisphenol A, glycerin, sorbitol, 1,4-sorbitan, trimethylolpropane, etc. Among these, preferable alcohol monomers include bisphenol A derivatives, in particular, polyoxypropylene (2.2)-2,2-bis(4-hydroxyphenyl)propane, polyoxyethylene (2.0)-2,2-bis(4-hydroxyphenyl)propane, etc.
With respect to monomers constituting the second binder resin in the present invention, trivalent or more monomers (including acid monomers and alcohol monomers) are used, preferably, at a rate of 3 to 50 mole %, more preferably 5 to 25 mole %, with respect to all the monomers constituting the resin. With respect to trivalent or more monomers, it is preferable to use trivalent acid monomers from the viewpoint of costs. In the case of preparation of a negatively chargeable toner, it is advantageous to use trivalent acid monomers from the viewpoint of chargeability. With respect to the trivalent acid monomers, among the above-mentioned compounds exemplified as acid monomers, those compounds having three carboxylic groups are preferably used.
In particular, the preferable first polyester resin is a linear polyester resin formed of a divalent acid monomer and a divalent alcohol monomer. The preferable second polyester resin is a non-linear polyester resin formed of a divalent acid monomer, a trivalent acid monomer and a divalent alcohol monomer.
In the present invention, upon the application of the first binder resin and the second binder resin as the binder resins, another resin different from the first binder resin and the second binder resin may be mixed therewith. With respect to another resin, not particularly limited as long as it has a compatibility with the first binder resin and the second binder resin, any of the above-mentioned resins exemplified as the binder resin may be used. An amount of use of this resin is preferably set to not more than 10% by weight with respect to the mixed binder resin formed of the first binder resin, the second binder resin and this resin.
In the binder resin of the present invention, from the viewpoint of the light-transmitting properties of the toner for OHP, the low-temperature fixing properties and the pulverizing properties of the resin, the insoluble content of tetrahydrofuran (hereinafter, referred to as THF insoluble content) is preferably set at not more than 5% by weight. In the binder resin of the present invention, the smaller the THF insoluble content, the better; and it is most preferable to set it to 0% by weight. In the present specification, the THF insoluble content is an insoluble content (% by weight) remaining on filter paper (No. 5B made by Advantech Co., Ltd.) when a sample (2.0 g) is dissolved in THF (250 ml) at normal temperature, and allowed to stand.
With respect to colorants contained in the toner of the present invention, not particularly limited, conventionally known pigments and dyes in the field of electrophotography may be used. Examples of them include carbon black, aniline blue, Chalco Oil Blue, chrome yellow, ultramarine blue, DuPont Oil Red, quinoline yellow, methylene blue chloride, copper phthalocyanine, Malachite green oxalate, Lump Black, Rose Bengal, C.I. Pigment Red 48:1, C.I. Pigment Red 122, C.I. Pigment Red 57:1, C.I. Pigment Red 184, C.I. Pigment Yellow 97, C.I. Pigment Yellow 12, C.I. Pigment Yellow 17, C.I. Solvent Yellow 162, C.I. Pigment Blue 15:1, C.I. Pigment Blue 15:3, etc.
The content of the colorants is not particularly limited; and, in general, it is preferably set to 2 to 10 parts by weight with respect to 100 parts by weight of the binder resin.
In the present invention, from the viewpoint of the dispersing properties of the colorant in the toner particles, it is preferable to preliminarily disperse the colorants in a resin having compatibility with the binder resin to be used so as to form a master batch. More specifically, a resin that has compatibility with the binder resin to be used is mixed therewith preferably at a weight ratio of the first binder resin and the colorant (resin/colorant) of approximately 100/15 to 100/50. After melted and kneaded, the mixture is cooled and pulverized to give a master batch. The master batch having passed through a mesh of 0.5 to 4.0 mm is preferably used, and the amount of use is set so as to contain the amount of the colorant that is maintained in the above-mentioned range.
A desired mold-releasing agent, charge-control agent, etc. may be properly incorporated with the toner of the present invention.
The mold-releasing agent is not particularly limited, and examples thereof include: polyethylene wax, oxdized-type polyethylene wax, polypropylene wax, oxdized-type polypropylene wax, carnauba wax, sazol wax, rice wax, candelilla wax, jojoba oil wax, beeswax, etc. A content of the mold-releasing agent is not particularly limited; and in general, it is preferably set in the range of 0.5 to 7 parts by weight with respect to 100 parts by weight of the binder resin.
To the toner of the present invention, a charge-control agent may be added on demand so as to further stabilize its chargeability. With respect to the charge-control agent, not particularly limited, generally known negative charge control agents that control the toner to have negative chargeabllity may be used. Examples thereof include: a metal complex of derivatives of salicylic acid, a calix arene compound, an organic boron compound, a fluorine-containing quaternary ammonium salt compound, a monoazo metal complex, an aromatic hydroxycarboxylic acid-based metal complex, an aromatic dicarboxylic acid-based metal complex, etc. Among these materials, those having no color (white color) are preferably used for color toners. A content of the charge-control agent is not particularly limited; and in general, it is preferably set in the range of 0.5 to 5 parts by weight with respect to 100 parts by weight of the binder resin.
With respect to the magnetic particles, iron particles, iron oxide particles, ferrite, nickel, etc. may be used. A content of the magnetic particles is not particularly limited; and in general, it is preferably set in the range of 0.5 to 10 parts by weight with respect to 100 parts by weight of the binder resin.
The toner of the present invention may be prepared by using a conventionally known method, for example, a pulverizing method, an emulsion dispersing granulation method, etc. From the viewpoint of the easiness in production and the productivity, it is preferable to adopt the pulverizing method. In the case of the application of the pulverizing method, the toner of the present invention is obtained, for example, in the following processes: The above-mentioned first binder resin and the colorant, and if necessary, the mold-releasing agent and the charge-control agent, are mixed together, and the mixture is fused and kneaded, and then cooled, coarsely pulverized, finely ground, and classified to obtain the toner. The resulting toner of the present invention is preferably controlled to have a volume-average particle size of 4 to 10 xcexcm.
Moreover, to the toner of the present invention, an externally additive agent and a cleaning-assist agent may be added and mixed. With respect to the externally additive agent, examples thereof include: silica fine particles, titanium dioxide fine particles, alumina fine particles, magnesium fluoride fine particles, silicon carbide fine particles, boron carbide fine particles, titanium carbide fine particles, zirconium carbide fine particles, boron nitride fine particles, titanium nitride fine particles, zirconium nitride fine particles, magnetite fine particles, molybdenum disulfide fine particles, aluminum stearate fine particles, magnesium stearate fine particles, zinc stearate fine particles, etc. These fine particles are preferably subjected to a hydrophobicizing process by using an agent, such as a silane coupling agent, a titanium coupling agent, a higher fatty acid and a silicone oil. A amount of use of the externally additive agent is preferably set in the range of 0.1 to 3.0% by weight.
Mith respect to the cleaning-assist agent, various kinds of organic fine particles, such as styrenic fine particles, acrylic fine particles, methacrylic fine particles, benzoguanamine, silicone, Teflon, polyethylene and polypropylene, which have been granulated by using a wet-type polymerization method, such as emulsion polymerization, soap-free emulsion polymerization and nonaqueous dispersion polymerization, or a gaseous phase method. These organic fine particles may be used alone, or may be used in combination with the externally additive agent.
The toner of the present invention can be applied to both of a one-component developer that does not use a carrier and a two-component developer that is used together with a carrier; however, it is preferable to apply it to the two-component developer. With respect to the carrier used together with the toner of the present invention, any of known carriers may be used: Examples thereof include:
carriers made of magnetic particles such as iron particles and ferrite, coat-type carriers formed by coating the surface of magnetic particles with a coating agent such as a resin, and dispersion-type carriers formed by dispersing magnetic fine particles in a binder resin. Any of these carriers may be used. Carriers preferably used in the present invention have an average particle size of 20 to 70 xcexcm, preferably 30 to 60 xcexcm.